The present invention relates to a vacuum-operated brake power booster with a booster piston sealed relative to the booster housing, and a piston rod coupled to a braking pedal for the actuation of a double valve through which a working chamber of the brake power booster is connectible either to vacuum or to a higher pressure differential. A a first valve is formed by a valve seat at the control housing associated with the booster piston, a poppet valve is pre-loaded in the direction of the valve seat, and a second valve is formed by the same poppet valve and a valve piston connected to the piston rod. The control housing is provided with an aperture protruding through which is a transverse member which confines the axial movability of the valve piston relative to the control housing. The transverse member is provided with a fork portion and at least one limb, which abuts against a stationary part, and the fork portion engages in an annular groove of the valve piston with clearance.
A vacuum-operated brake power booster of the same general type is known by German printed and published patent application No. 31 01 795 A1. This reference discloses a transverse member confined in conjunction with a rod. Return movement is provided through the intermediary of the rod to limit the return movement of the valve piston as the valve piston returns to its return position. In the reference booster, the transverse member is displaceably arranged on the rod and is capable of moving into contact with a shoulder formed at the rear end portion of the rod and with the valve piston when the control housing returns to the return position. The radially inner end of the transverse member is enclosed by an annular groove of the piston, which annular groove is shaped in the valve piston. The transverse member loosely protrudes through an opening situated in the control housing, the opening acting as a part of the connecting channel between the vacuum chamber and the working chamber. This known vacuum-operated brake power booster is disadvantageous in that the transverse member confining the movement of the valve piston acts together with a rod which bears against the housing shells of the booster, as a result whereof the assembly of the transverse member is comparatively difficult and time-consuming. Another disadvantage lies in that the transverse member is designed such that it cannot be used in a brake power booster withou special rods.
Furthermore, a fluid pressure-operated servo motor for a brake power generating device is known by German printed and published patent application No. 30 42 096 A1. In this reference, a sequential control valve includes a vacuum valve seat at the wall element and a displaceable element which is slidingly seated in the wall element and which is functionally connected to a braking pedal actuatable by an operator. An annular valve seat at the displaceable element is movable relative to the vacuum valve seat. A poppet valve element is arranged at the wall element and is normally urged by means of a spring force into abutment with either the vacuum valve seat or the annular valve seat. A transverse member is seated with clearance both in the displaceable element and in the wall element. The member can move into abutment with the housing such that different positions of the vacuum valve seat and the annular valve seat are provided when the movable wall element and the transverse member abut on the housing. As a result, in the open position of the sequential control valve, the vacuum valve seat is lifted by a minimum amount off the poppet valve element.
The transverse member of the reference includes on its outside an annular groove and the wall element has a radial recess. The transverse member is equipped with a fork portion and a finger such that the fork portion is seated with clearance in the annular groove in the displaceable element and in the radial recess in the wall element so that it is capable of moving relative to said elements in the axial direction around the axial length of the annular groove and/or the axial dimension of the radial recess, the finger being arranged such that it can move into abutment with the inside of the housing.
The disadvantage of this known fluid pressure-operated servo motor lies in that the attachment of the transverse member is via a tab bent at right angles, the tab engaging in a recess provided in the hub of the booster piston. The tab is secured against radially outward slippage by means of the radially inside bead of the diaphragm. On removing the transverse member, e.g. in the event of repair work, it is therefore necessary in each case to remove the diaphragm from the hub so that the tab of the transverse member is released and the transverse member can be extracted from the hub.